Transmission dynamometer



sept. 16, 1941. N. s.' Mura 2,256,406;

l TRANSMISSION DYNAMQMETER Q original Filed Feb. 15, 19;' 'rj s sheets-sheet 1 I l l sept. 16, A1941. f N. s. Munl 2,256,406

original Filed Feb'. `15, l19s? 3 sheets-sheet 2 4s'-sh'eje"LSL-sheet v,

TRANSMISSION DYNAMOMETER y original 'Fired 515,515, 1955?",

Patented Sept. 16, 1941 UNITED 'STATES' PATENT OFFICE Divided and this application August 9, 1938, Serial No. 223,987. In Great Britain February 21, 1936 8 Claims.

This invention relates to transmission dynamometers for measuring thrust in a power transmission system, and the present application is divisional from the present applicants Letters Patent of the United States of America No. 2,173,039, which describes features similar to those of the present invention applied to a torsion dynamometer for power transmission mechanism.

One object of the invention is to provide a transmission dynamometer capable of measuring the true mean value of thrust transmitted irrespective of transient variations or oscillations of such thrust.

Another object of the invention is to provide a thrust dynamometer comprising two springs or the equivalent forming part of a power transmission system and connected in series with one another between the driving and driven members of the system, a device for damping oscillations of one oi the springs, and means for measuring the deflection of the other spring.

It is to be understood that by the term "spring is meant any member which is caused to deiiect or otherwise vary its shape or size in accordance with the force exerted thereon due to the power transmitted by the system ofrwhich such member forms a part. Thus for example in a traction dynamometer or a thrust-meter the spring may consist of a disc, diaphragm or extensible member, the deflection, extension or contraction of which can be measured or damped as the case may be.

A further object of the invention is to provide electro-magnetic apparatus for measuring small relative movements between two elements which constitute part of a spring and are caused to move relatively to one another bythe applied force. The relative movement which it is desired to measure is usually only one component of the various possible relative movements which can take place between the two elements, and for convenience will hereinafter be referred to as the selected component of movement.

This feature of the invention is concerned with electro-magnetic measuring apparatus of the kind in which the selected component of movement is caused to vary the reluctance of a magnetic circuit, as by varying the length of one or more air gaps therein, the resultant change in the magnetic flux being arranged to influence one or more suitably disposed coils connected to a measuring instrument or the equivalent.

arrange a pair of similar normally balanced magnetic circuits partly on one element and partly on the other element so -that the ux density or distribution of flux or both is varied in both circuits by relative movement of the elements, the disturbance of equilibrium of the magnetic circuits being measured by a suitable l indicating instrument. It has also been suggested to provide in such an arrangement, a similar second pair oi magnetic circuits, the equilibrium of which can be disturbed by hand or otherwise so that by connecting the two pairs in opposition to an indicating instrument, the instrument shows by a null indication when the equilibrium 'of the two pairs of magnetic circuits is disturbed by the same amount. The disturbance of the equilibrium of the second pair of circuits required to produce the null indication thus affords a direct measure of the disturbance of the equilibrium of the first pair by the movement to be measured.

Diiilculties have been experienced with apparatus of the aforesaid kind owing to inaccuracies which are introduced when there are components of movement other than the selected component. These y difficulties become particularly marked when the apparatus is applied to thrust-meters of the transmission type which operate'at relatively high speeds-for example when employed in aircraft engines-and there are appreciable relative movements of the two elements in several directions due, for example, to shaft whip or vibration, axial misalignment or other causes. An object of the present invention is to provide improved electromagnetic measuring apparatus of the above kind from which it is possible to obtain readings of the selected component which are substantially independent of or unaffected by other components of relative movement of the two elements concerned.

Yet another object of the invention is to provide a magnetic measuring apparatus in which -three or more magnetic systems or circuits are mounted'with part of each magnetic system on one of the relatively movable elements and part of each system on the other element so that the several systems are symmetrically disposed with -respect to the selected component of movement and relative movement'I of the two elements varies the magnetic reluctance of the several sys tems, means being provided responsive to changes in the reluctance of the systems and so arranged that changes in reluctance of the several systems produced by the selected, compo- It has been proposed, in such apparatus, to nent of movement are additive whilst changes be provided with4 multiple air gaps and coils responsive to variations of magnetic flux in the associated system, the coils of the several sys- `tems being so interconnected that voltages induced therein by variations of magnetic ilux due to changes in the lengths of the air gaps, are additive when the changes are produced by the selected component of movement and substantially balance out when produced by other components of movement.

When the relative movement to be measured is al translational one, the several systems are symmetricallydisposed about an axis parallel to l5 the direction of movement and in each system there are, between the'parts on the two elements,

at least one air gap parallel to the axis and one air gap perpendicular to the axis (or an air gap the axis), the coils being so interconnected that changes in the lengths of the air gaps parallel to the axis are additive whilst changes in the voltages due to changes in the lengths of the other 'air gaps substantially balance out.

In the above definition of the invention and throughout the specification air gaps are specified in terms of the .direction of the length of the having components parallel and perpendicular to ments in an aircraft engine transmission system,

Figure 5 is a front elevation, partly broken away, of part of the measuring apparatus,

Figure 6 is a side elevation, partly in section, of the apparatus shown in Figure 5,

Figure 7 is a wiring diagram showing the electrical connections of the apparatus shown in Figures 1 to 6, and

Figure 8 is an elevation, partly-in section. of a modified form ofthe measuring apparatus shown in Figures 5 and 6.

A construction of dynamometer which is suitable for measuring translational movements such as the thrust of a rotating shaft, as in a thrustmeter, ordirect-tension or compression, as in a traction dynamometer, is shown in Figure 1 as employed for measuring the thrust developed by an airscrew. In the arrangement shown in this ligure, the drive from the engine shaft As is transmitted through a disc 0 which may be solid or may consist ofk spokes which are flexible in directions parallel to the axis, having its hub Ol connected to the shaft Ae and its rim O2 to a ilangeon the airscrew boss A, this boss being mounted on ball or roller bearings on an extension of the shaft A. 'I'he disc O should be sufgap measured along the lines of magnetic flux which pass across it. Thus, for example, an 30.

axial air gap is one in which the length, as defined above, lies along a line parallel to the axis, whilst a radial" airgap is one in which the length of the air gap lies along a radius of a circle.

Preferably each magnetic system comprises a pair of similar normally balanced magnetic circuits arranged so that the selected component of movement causes a change in the length of an air gap in each circuit, the changes in the air 40 gaps in the two circuits being in opposite senses.

In one convenient construction each system comprises a substantially U-shaped main body or core of magnetic material connected to one element and an armature disposed between the limbs of the core and connected to the other elements. Preferably the core is so magnetised as to form two magnetic circuits one of which includes the air gap between the armature and one of the limbs of the core and the other the air gap between the armature and the other limb of the core', an air gap formed between the armature and the yoke connecting the limbs of the core being common to both magnetic circuits.

Preferably each magnetic system is provided with two coils respectively responsive to changes of magnetic flux in the two circuits of the system, the two coils being connected in opposition and the opposed pairs of coils of the several systems being connected in series.

The following is a description, by way of example, of several convenient constructions ofL transmission dynamometer and electromagnetic measuring apparatus according to this invention ilciently flexible for the rim O2 to move relatively to the hub Ol in a direction parallel to the axis (of the shaft A8) by an amount depending upon the axial thrust. The electromagnetic apparatus thus has to measure relative axial movements between the rim Oa and the hub O1.

This apparatus comprises a number of individual magnetic systems which are disposed in planes containing the axis of the shaft. Each system comprises a T-shaped armature P disposed between the limbs Q1 and Q2 of a U-shaped core Q so that there are three air gaps P1, P2, PJ in each system. The several magnetic systems are symmetrically arranged about the axis oi the disc O so that in each system the two air gaps P1 and respectively liein two planes parallel to one another and perpendicular to the axis, their'lengths being parallel to the axis, whilst that of the air gap P3 is perpendicular to the axis, i. e. substantially radial. The armatures P of the several systems are mounted on an annular ring P'4 which is rigidly connected to the rim Oa of the disc O, whilst the cores Q are mounted on a hub or spider Q3 keyed to the shaft A so as to be, in effect, rigidly connected to the hub'O1 of the disc O. Stops (not shown) may be provided to limit the relative axial movement of the boss and the hub.

When, due to airscrew thrust, the disc O ilexes and the rim O2 moves relatively to the hub O1, the lengths of the air gaps Pl and Pz change, the gap P1 of each system increasing in length and the gap P2 decreasing, the-lengths of the radial air gaps P3 remaining unaltered. If, however, there is relative movement of the rim O2 and the hub O1 in planes perpendicular to the axis, the lengths of the radial gaps P:l vary. Coil units E, each containing a primary coil E1 and a secwith reference to the accompanying drawings, in

which Figure 1 is a sectional side elevation of a dynamometer for thrust measurements in an aircraft engine transmission system,

. Figure 2 is a sectional side elevation of the dy- 70 lnamometer shown in Figure 1 provided with a elevations (partly in section) of another construction of dynamometer for thrust measureondary coil E, are mounted around the air gaps P1 and P2 of each-system, and the coils of the several systems are so connected that the secondary output voltage depends on the mean variation of the lengths of the air gaps P1 and P2 and is substantially independent of variations in the lengths of the radial air gaps P3.

The several systems are magnetised by passing an intermittent or alternating current from an appropriate generator, indicated at G in Figure 7, through coils El (Figure 7), these coils being soarranged that each amature P formspart of one magnetic system and each system has two magnetic circuits. There are thus a number of magnetic systems corresponding to the number of armatures Q symmetrically disposed around the shaft A2.

In each magnetic system the flux in one magthe system is similar to that described above but includes the axial air gap P2 at the other end of the armature P.

It is to be noted that the radial air gap-P2 and the radial member of each armature P form part of both the magnetic circuits of the associated system. Accordingly the several magnetising coils E1 are connected to the generator G in series with one another in the appropriate sense for the magnetic fluxes of two magnetic circuits which in part have a common path to be in the same direction in such common path. The initial lengths of the air gaps P1, P2 and P2 and the excitation are so chosen that the ux densities in the armatures P and Q (even when there is maximum axial movement and one air gap P1 or P2 is very short) do not approach the saturation value. Thus,`the variations in the mean flux in the two magnetic circuits of a system are approximately directly proportional to the variations in the lengths of the axial air gaps of the system.

Each of the magnetic circuits is provided with a coil E2 (Figure 7) which is so mounted that it is responsive to changes in the magnetic flux in the circuit, this coil being hereinafter referred to as the secondary coil to distinguish it from the magnetising or primary coil of the circuit. The primary and secondary coils E1 and E2 of each magnetic circuit are preferably, as shown in Figure 1, mounted together so as to form a single tubular coil unit E2 surrounding the air gap P1 or P2 of the associated magnetic circuit. The primary and secondary coils E1 and E2 may be wound side by side as indicated diagrammatically in Figure 7, either on a single former or on separate formers. Preferably, however, alternate layers of primary and secondary turns are wound on a single former or each coll is made of a series of flat coil units, the units of the primary and secondary windings being arranged alternately, or' any other convenient winding arrangement is adopted so as to obtain a high degree of inductive coupling between the primary and secondary windings. If desired any convenient form of metallic screening may be employed between the primary and secondary windings so as to reduce the capacity coupling between them.

The two secondary coils E2 of each magnetic system are connected in opposition so that when the two air gaps P1, P2 of the systems are equal, there is no secondary output. When there is relative axial movement between the hub O1 and the rim O2, the increase in length of one air gap, say the gap P1, and the corresponding decrease in the other air gap (P2) of each system, disturbs the balance of the secondary voltages induced in the coils E2 (due to the current flowing in the primary windings E1) and the outof-balance voltage produced is a measure of the mean axial movement and, in the appropriate cases, of the ,thrust transmitted through the spring. The opposed pairs of coils E? of the several systems are. as shown in Figure '1, connected in series so that the out-of-balance voltages are additive and thus produce a totaloutput voltage which is a measure of the mean variation in length of the axial air gaps P1, P2 ofall the systems.

When there are radial oscillations in the transmission system or there is slight axial mlsalignment between the engine shaft A2 and the airscrew boss A. the radial air gaps P2 will vary correspondingly and thus produce alterations in the out-of-balance voltages derived from the individual pairs of coils E2. Since, however, the magnetic systems are symmetrically disposed about the axis of the shaft A2, any radial displacement which causes an increase in the secondary'output voltage from the coils of the system or systems on one side of the axis will cause a corresponding decrease in the output voltages derived from the coils of the system or systems on the diametrically opposite side of the axis. Thus. by reason of the series connection of the coils E2, the variations due to radial displacement will balance one another so that the resultant v output voltage is independent thereof. Thus, the

apparatus is rendered substantially independent of relative movements of the hub O1 and the rim O2 other than relative axial movements.

Frequently, however, it is desirable to reduce as far as possible'current flow in the secondary coils and associated circuits. This is particularly so in apparatus having rotating magnetic systems to which connections have to be made through slip rings. Even when, as is usual in such an arrangement, variations due to brush vibration are reduced by providing each slip ring with three or more brushes spaced around its periphery, the current flowing causes voltage drops which vary with the output voltageand consequently with axial movement. Thus the readings must be corrected and the corrections necessary are diiiicult to make, particularly when measurements are made at varying speeds.

Accordingly measurement of the relative movement is preferably effected by a null deflection" method of the known general type which is referred to above. To this end the output circuit from the secondary coils of the several magnetic systems of the apparatus shown in Figure 1,

A which, for convenience of description will be referred to as the transmitter, is connected to the output circuit of a receiver which is shown in Figures 5 and 6 and comprises a set of magnetic systems similar in characteristics to the magnetic systems of the transmitter. The receiver is provided with a main core J having radial arms J1 with poles J2 and J2 corresponding to those of the cores Q of the transmitter. The receiver is provided with T-shaped armatures K similar in dimensions and shape to the armatures P so that there are circumferential air gaps F2 and F1 respectively between the pole J2 and one end K2 of an armature and between the pole J2 and the other end K2 of the same armature and a radial air gap F5 between the upright member K1 of the armature and the part of the core J between adjacent arms J1.

The magnetic systems of the receiver are arranged so that their air gaps F2 and F1 can be varied by rotating a hand wheel or other manually operable control device. Thus, as clearly shown in Figure 6, the receiver has a main tubular supporting shaft L on which is mounted a sleeve L1 capable of sliding along the shaft but prevented from rotating thereon by a key. The sleeve Ll has at one end (the front end) a ange L2 connected through a ball bearing to an externally screw-threaded cylindrical member L3 connected ,at its front end to a hand wheel L4. The screwthread on the member L:I engages with an internal screwthread in a sleeve Ll which is fixed on arms L7 extending axially from the end of a tubular member L fixed to the rear end of the supporting shaft L. When the hand wheel L4 is rotated, the sleeve La is rotated relatively to the fixed member L and the engaging screwthreads cause longitudinal movement of the member l'..3 and a corresponding movement of the sleeve L1 connected thereto, the whole arrangement constituting a reduction drive whereby a large rotational movement of the handwheel L4 produces a relatively small longitudinal movement of the.l

sleeve L1.

Mounted on the rear end of the sleeve L1 is a supporting spider J4 for the magnetic core J. The spider J4 is provided with slots through which the arms L" pass, the arrangement being such as to permit the requisite rocking movement of the spider J4 about the axis of the shaft L for varying the air gaps F3 and F4. Axial movement of the spider J4 on the shaft L is prevented by thrust bearings arranged between the spider J4 and radial flanges La and L9 on each side thereof, the flange LB being .formed integral with the member L4 and the fiange L9 screwed on to the ends of the arms L". 'I'he hub of the spider J4 engages the sleeve Ll in such a manner (e. g. as by inclined cam surfaces or a quick-pitch screwthread arrangement), that longitudinal movement of the sleeve along the shaft L causes rotational or rocking movement of the spider J4. Owing to the reduction drive referred to above a relatively large movement of the hand Wheel L4 produces only a small rocking movement of the spider` J4 and the core J carried thereby. The armatures K are carried by a plate K4 fixed on the member L6 and having a pointer K5 which cooperates with a disc-like scale L10 arranged to move with the hand wheel L4. In order to ensure positive movement of the spider J4 as the sleeve L1 moves longitudinally, the cooperating parts (e. g. cams and slots or screwthreads of the quick-pitch type) on the hub of the spider J4 and on the sleeve I..1

are maintained in engagement by springs J5 connected between the plate K4 and pins J8 carried on the spider J4 and passing through slots in the plate K4, the pins J4 also acting as stops to limit the permissible movement of the core J. The hand wheel L4 is protected by a disc-like shield L1l keyed to the shaft L.

The receiver is provided with primary and secondary coils K6 and K7 (Figure 7) which are preferably, as in the transmitter, arranged as single tubular coil units K8, there being one of such units mounted around each of the air gaps F3 and F4. As shown in Figure 7, the primary coils- K8 of the receiver are energised from the source G in series with the primary coils E1 of the transmitter. The secondary coils K'I of the receiver are connected so that the two coils of a system are in opposition and the several pairs are connected in series. The output circuits from the transmitter and the receiver are connected in opposition in series with a sensitive electrical measuring instrument G1 (Figure '7) the impedance of which should be matched with that of the associated circuit so as to obtain maximum sensitivity. The measuring instrument Gfx is preferably, as shown, ofthe dynamometer type When there is no movement to be-measured I and the receiver is in its mid position, there .is

no output from either the receiver or the transmitter and the pointer of the instrument G4 (which preferably has a central zero position so as to give a directional indication) remains in the zeroV position. When the thrust is transmitted through the spring, there is relative movement between the hub O1 and rim Oa and the resultant output voltage of the transmitter secondary coils causes a deflection of the pointer of the instrument G. The hand wheel L4 of the receiver is then operated to alter the air gaps ll and F4 of the receiver in the appropriate direction until electrical balance is restored and there is no deflection.

'I'he movement of the receiver air gaps F3 and F4 to restore balance is an indication of the mean movement of the transmitter air gaps F and 1E'l due to the mean relative movement between the hub 01 and rim O2, and thus the movement ofl the hand wheel L4 which is required provides an indication of the mean movement to be measured. The movement of the hand wheel is indicated by the reading on the disc L10 opposite the pointer K5, any suitable Vernier or other device (not shown) being fitted, if required.

-The source of alternating current G used for energising the primary coils E1 and K should have a frequency appreciably above the frequency of any mechanical oscillations likely to occur in the power transmission system on which measurements are being made, this A. C. frequency preferably -being at least ten times the highest frequency of any appreciable vibrational movement likely to occur. 'I'he exciting current may be supplied from any suitable source having its frequency stabilised and capable of supplying a constant voltage. This source may, for example, comprise a permanent magnet alternator or a valve oscillator and should preferably have a sinusoidal voltage output. When an intermittent D. C. is used, instead of A. C., it may be supplied from a battery or other suitable source through a rotary commutator driven at the speed necessary to produce the desired impulse frequency. a

'Ihe coil windings and the initial settings of the air gaps are preferably chosen to suit the operating characteristics and other properties of the transmission system and/or shaft or spring with which the apparatus is being used. It has been found that the accuracy of the readings is increased if a step-up ratio is used between the primary and secondary coils (ElzEI and KMK?) as this increases the secondary voltage output for a given change in air gap length.. Furthermore it is desirable so to choose the transformation ratios of the two pairs ot coils `on each magnetic system as to compensate for non-linear characteristics in the two magnetic circuits of the system in such a way that the resultant output voltage fromthe two opposed coils of a system has a substantially linear characteristic, i. e. is directly proportional to the changes in lengths of the air gaps (P1 and P2) provided that the apparatus is arranged so that the movements to be measured always take place in one direction from the mid position in which the air gaps are equal. The

winding ratios can, when necessary, be selected so as to compensate for non-linearity of the thrust-deection characteristic of the part of the transmission system across which the`apparatus is connected. From the foregoing it will readily be appreciated that the characteristics of the improved measuring apparatus according to this invention can be matched to suit any desired type of spring, and that when so matched the apparatus gives an accurate measure of the mean value of a uctuating thrust.

Figure 8 shows an alternative form of receiver, which is preferably employed, in which the magnetic systems are similar to those of the arrangement shown in Figures and 6 but which differs from that shown in these gures in the means for effecting relative movement between the core and the armatures.

Thus, as shown in Figure 8, in which parts corresponding to parts of theapparatus shown il: Figures 5 and 6 are given the same reference letters, the core J is supported on a plate l rotatably mounted on ball bearings on a fixed shaft l1 whilst the armatures K are carried on a fixed plate lc4 keyed to the shaft l1. A ball thrust bearing (not shown) is provided between the plates Z and 7a4, the arrangement being such as to prevent all relative movement of the two plates other than angular or rocking movement.

Rocking movement of the plate l relative to the plate 11:4 to. effect the desired alteration in the lengths of the air gaps ZF and F4 is effected by means of a radial arm P extending from the plate l and moved relatively to a xed member k5 on the plate 1c4 by slow motion screw mechanism. This mechanism comprises a screwthreaded rod Z3 having one end xed to the arm l11 and the other end engaging in an internally screwthreaded sleeve Z4. This sleeve is also externally screwthreaded to engage in an internally screwthreaded collar l5 rigidly connected to the plate k4. The pitches of the screwthreads on the inside and outside of the sleeve l4 differ from one another and, moreover, are of the same hand. Thus at each rotation of the sleeve 14, the arm l2 is moved towards or away from the member Ic5 by an amount depending on the difference in the pitches of the two screwthreads, movement of the arm l'1 effecting a corresponding rocking move-` ment of the plate l and the corev carried thereby. A hand wheel Z6 is provided for rotating the sleeve Z4 and the amount of movement of the arm Z2 is recorded by a counting device l" which indicates the number of revolutions of the sleeve Z4 in conjunction with a disc l8 which is attached to the hand Wheel l6 and cooperates with a pointer Z9 ixed to the member k5 to indicate fractions of a revolution of the hand wheel l*1 and sleeve 14.

In other respects this receiver is similar to that shown in Figures 5 and 6 and thus it need not be described in further detail.

It will be appreciated that this operating mechanism provides a very accurate means of obtaining and measuring very small movements of the core J relative to the armatures K and that it avoids all backlash or variable factors which sometimes arise in the arrangement shown in Figures 5 and 6 even when 'this apparatus is, as described, provided with the springs J5.

If desired the construction illustrated in Figure 1 may be provided with a damped spring to abstruction of Figure 1 are given the same reference letters but small instead of capital letters, an airscrew boss n is supported on bearings on an extension of the engine shaft n1 in such a manner that it is capable of moving axially on this shaft, and two springs, which are flexible in an axial direction, are arranged on the shaft n1 between the engine (not shown) and the boss n.' The rst spring comprises a disc o which may consist of spokes which are flexible in directions" parallel to the axis having a hub o1, keyed to the shaft n1, and a rim o. 'I'he rim o2 is bolted directly to the rim o3 of another similar flexible disc o4 having a hub o5 provided with a flange 0 connected, through a hub (ls forming part of the measuring apparatus to be described below, to a ange n1 on the airscrew boss n. The hub o5 may be keyed to the shaft n1 so that the drive is transmitted directly to the airscrew boss, or this hub may be free to rotate on the shaft so that the drive is actually transmitted through the discs o and o4. In either case, however, the hub o5 is free to move longitudinally on the shaft and its normal position is such that, as shown in Figure 2, it does not engage either of the rings n3 and n4 provided on the shaft on each side of the hub o5 to linut its axial movement. During operation the airscrew thrust tends to cause movement of the hub ofs relative to the xed hub o1, the thrust being transmitted through the discs o4, the rims o3 and o1l and the disc-o to the xed hub o1. 'I'he discs o4 and o thus deflect axially by an amount dependingy upon the airscrew thrust, this deflection taking place irrespective of whether or not the hub o5 is actually keyed (by a feather key or the equivalent) to the engine shaft n1. 'I'he axial vibrations or oscillations of the disc o are damped by a. device consisting of two annular rings c11 and c1 respectively connected to the engine shaft n1. and the rim o2. The rings e9 andc1 have their cooperating surfaces in the form of cylinders concentric with the axis and separated by a friction lining c11. The shaft n1 supports a ring Y1 carrying slip lrings Y2 provided with brushes Yi held by a ring Y4 which may be bolted to the engine casing.

The apparatus for'measiu'ing the deflection of the disc o4 may be of any convenient type but is preferably similar to that employed in the construction shown in Figure l which is described above.

When, due to airscrew thrust, the discs o and o4 iiex, there is relative movement between the hub o5 and the rim o3 and also between the rim o2 and the hub o1. Deflectionsof the disc o due to, for example, transient oscillations are adequately damped by vthe friction between the portions of the lining c11 carried by the rings c9 and c1, whilst deflection of the disc o4 causes the lengths of the air gaps p1 and p2 to alter, the gap p1 of each system increasing in length and the gap p2 decreasing. The lengths of the radial air gaps p3 remain unaltered. If there is relative sorb axial oscillations, such a construction being movement of the rim o3 and the hub o5 in planes perpendicular to the axis of the shaft n1, the

transmitter) is balanced against that from an exactly similar arrangement of magnetic systems (the receiver) which are adjusted by a hand controller, the movement of the hand controller required to obtain balance being measured.

. Another construction, also suitable for thrust or traction measurements or for measuring other translational movements, is shown invFi'gures 3.

and 4 as employed for measuring airscrew thrust. In this arrangement the drive is transmitted from the engine shaft A11 to an airscrew boss A11 mounted in ball bearings on an extension of the shaft through a radial spoke spring having a hub R connected to a rim R1 by spokes R2 which (as most clearly shown in Figure 4) are relatively stiff in directions perpendicular to the axis and flexible in directions parallel to the axis. This drive is thus similar to that shown in Figure 1 in which the rim R1 moves relatively to the hub R in an axial direction due to airscrew thrust, the spokes R1 being sumciently stiff to prevent relative angular movement between the hub R `and the rim R1 due to the torque transmitted.

connected between the hub R and the rim R1.

'I'he arrangement on the same side of the spokes R1 as the engine comprises an annular core S with salient poles S1 surrounding a central ringlike armature S1'. The arrangement on the other side of the spokes R1 has a similar core S3 with radial spokes S4 and a central armature S11. The armatures S2 and S11 are supported between nanges on axial extensions of the hub R so that they are coaxial with the shaft A11 and are parallel to one another. The two cores S and Si1 are rigidly supported on flanges 1'1.3 and R1 attached to or formed integral with the rim R1 on each side thereof, there being air gaps T between the poles S1 and the armature S2 andair gaps T1 between the poles S1 and the armatures S1. Thus, the selected component of movement, i. e. axial movement, between the rim R1 and the hub R, causes relative axial movement between the armatures S2, S1 and the encircling cores S and S1.

'I'he faces of the poles S1 and S1 of the cores and the cooperating surfaces of the armatures S1' and S1 are chamfered so that the air gaps T and T1 between the poles and the armatures are inclined to the axis at an angle, say such that when the armatures move axially, the lengths of all the air gaps T or T1 between each armature and its cooperating poles increase or decrease by the same amount, whilst when the armatures move in planes at right angles to the axis, the lengths of the air gaps on one side of the shaft A11 increase and those on the other side decrease. The lengths of the air gaps T and T1 of the two systems are inclined to the axis at angles which are equal but in the opposite sense. Thus, when there is relative axial movement between the cores and the armatures due to axial deflection of the spokes R1 produced by airscrew thrust, the air gaps, for example the air gaps T1 between the poles S4 and the armature Si. decrease, whilst the air gaps T between the poles S1 and the armature S1I increase.

The cores S and S1 are provided with primary windings U wound around the parts thereof between adjacent poles, the primary windings U being so connected in seriesthat when the coils are energised from a suitable A. C. source, each core S or S1, with its associated armature S2 or S1, is excited in such a manner that there are a number of magnetic circuits corresponding to the number of poles S1 or S4 formed in the core, each magnetic circuit extending from one air gap T or T1 (Figure 3) through the associated pole and the core to an adjacent pole, across the air gap of this pole to the armature and back through the armature to the first air gap.

The poles S1 of the core S are provided with secondary windings U1 and the poles S1 of the core S1 with secondary windings U1. 'I'he secondary coil U1 on each pole S1 is connected in opposition with the winding U1 on the corresponding pole S1 of the core S1, the pairs of oppositely connected secondary windings U1, U1 being connected in series to provide the output voltage. If desired, instead of connecting the corresponding secondary coils U1, U1 directly in opposition, all the secondary coils U1 on the poles S1 can be connected in series with one another and in opposition to the series-connected secondary coils U1 on the poles S1. Slip rings and cooperating brushes (not shown) are provided for making connection to the primary coils U and the secondary output circuit from the coils U1 and U1. It will be apparent from the arrangement of the connections that the magnetic circuits of corresponding poles of the two cores 8 and S3 of this construction correspond to the two magnetic circuits of each magnetic system of the other constructions described above. Thus when the armatures S1 and S*s are midway between the cores S and S1i so that the lengths of the air gaps T and T1, measured in a direction parallel to the axis, are equal. the outputs from the opposed secondary coils U1, U2 will substantially balance, whilst when there is relative axial movement between the armatures and the cores, an out-of-balance voltage will be produced proportional to such movement.

Should there be relative movement between the rim R1 and the hub R in planes at right angles to the axis of the shaft A11, i. e, components of movement other than the selected component, the lengths of the air gaps T and T1, as

measured in a direction perpendicular to the or by a null deflection" method as in the other constructions described, and since the circuit arrangements are generally similar to those described above, it is believed to be unnecesary to describe them in further detail. v

It will be noted that the improved electromagnetic measurlng apparatus according to this invention not only has the advantage that it enables a measurement to be obtained which is substantially independent of external or other vibrations of the system or apparatus to which it is applied, but also it is compact and can readilyv be tted to existing apparatus at. for example, shaft couplings. Furthermore, owing to its symmetrical form its application'does not disturb the balance of a rotating system nor has it itself any speed error due to centrifugal force, which, in many forms of dynamometer apparatus hitherto known, has to be corrected for in so far as this is possible. Yet another advantage of the apparatus is that it is relatively simple to use and does not require expensive and delicate` measuring instruments, this advantage being of particular importance when the apparatus is used in aircraft for obtaining values of thrust when ying.

It will be apparent that in the construction of dynamometer having two springs in series the clamped spring serves to .absorb or smooth out transient variations, for example, cyclic or other variations in the force transmitted due to the characteristics of the engine or other source of driving power or to the nature of the load on the transmission system. The consequent reduction in the amplitude of such variations not only facilitates the measurement of the true mean value of the deflection of the free spring but also simpliiles the construction and design of this spring. Another important advantage of this improved dynamometer, particularly when used in rotary machinery, is that it is symmetrical about the laxis of rotation and is independent of speed variations.

It is to be understood that the springs used will beso chosen with respect to the force to be transmitted and the general operating conditions that the exibility of each spring always conforms to predetermined calibrated values and that after detlection the spring across which measurements are made returns to its original position when the load is removed, without backlash.

What I claim as my invention and desire to secure by Letters Patent is:

l. In a thrust dynamometer for power transmission mechanism, the combination with two transmission elements of the transmission mechanism, of two resilient members connected-between tbe transmission elements so Athat the thrust to be measured is transmitted through them in series, damping means connected across one resilient member, and measuring means connected across the other resilient membeii t'o measure the deflection thereof.

2. In a thrust dynamometer for power transured is transmitted through said means and an electromagnetic measuring device connected to said elements to measure the relative axial movement between them and including a plurality of magnetic circuits symmetrically arranged about an axis parallel to the line of -the thrust to be measured and each including a U-shaped magnetic core and an armature arranged be.- tween the limbs thereof so as to leave an air gap between the armature and each limb of the core, the core and armature bein-g so carried respectively by the two transmission elements that relative axial movement will increase the length of one of the said air gaps and decrease that of the other.

5. In a thrust dynamometerfor power transmission mechanism, the combination with two transmission elements of the transmission mechanism of resilient connecting means connected between them so that the thrust to be measured is transmitted through said means and an electromagnetic measuring device connected to said elements to measure the relative axial movement between them and including a plurality of magnetic circuits symmetrically arranged about an axis parallel to the line of the thrust vto be measured and each including a U-shaped magnetic core and a 'l'shaped` armature situated within mission mechanism, the combination with two transmission elements of the transmission mechanism, of two resilient members connected between the transmission elements so that the thrust to be measured is transmitted through them in series, damping means connected across one resilient member, and measuring means connected across the other resilient member and including at least three magnetic circuits symmetrically arranged about an axis parallel to the line of the thrust to be measured and each having parts adapted to be relatively moved to vary the reluctance of the respective circuit in accordance with the deflection of the resilient member due to the transmitted thrust to be measured,

3. In a thrust dynamometer for power transmission mechanism, the combination with two rotating transmission elements, namely a driving member and a driven member, of two resilient members connected between the transmission elements so that the thrust to be measured is transmitted through them in series, damping means connected across the resilient member nearer to the driving member, and measuring means connected across the resilient member nearer to the driven member for measuring the deection thereof.

4. In a thrust dynamometer for power transmission mechanism, the combination with two transmission elements of the transmission mechanism of resilient connecting means connected between them so that the thrust to be measthe U-shaped core so that the head portion forms a pair of axial air gaps with the limbs of the core whilst the stem portion forms a radial air gap with the base of the core, the core and armature being so carried respectively by the two transmission elements that relative axial movement `will increase the length of one of the said axial air gaps and decrease that of the other.

6. In a thrust dynamometer for power transmission mechanism, the combination with two transmission elements of the transmission mechanism of resilient connecting means connected between them so that the thrust to be measured is transmitted through said means and an electromagnetic measuring device connected to said elements to measure the relative axial movement between them and including a group of at least three magnetic circuits symmetrically arranged about an axis parallel to the line of the thrust to be measured and each having parts connected respectively to the transmission elements so as to be relatively moved to vary the reluctances of the circuits in accordance with the axialmovement to be measured, a pair of stationary members relatively movablev about an axis, a manually operable device for producing such relative angular movement, means for measuring the latter, a second group of corresponding magnetic circuits having parts so carried by the stationary members that relative angular movement between the latter will produce changes of reluctance thereof corresponding to those produced in the first group by relative axial movement of the transmission elements, and means for detecting lack of balance between the reluctances of the two groups of magnetic circuits.

7. In a thrust dynamometer for power transmission mechanism, the combination with two transmission elements of the transmission mechanism, of two resilient members which are connected between the transmission members so prises inner and outer coaxial anhuli and a plurality of resilient spokes formed integral with both said annuli to avoid backlash tapering from both ends towards an intermediate point and shaped so as to be comparatively flexible as regards relative axial movement lbetween the annuli and comparatively .stiiT as regards relative angular movement about the axis, means for measuring the relative axial movement between the annuli, and damping means connected across the other resilient member.

NEIL SHAW MUIR. 

